Looking for excitement in the post-war world, Benoist joined the de Marçay car company as a test driver. He then moved on to Salmson and was very successful in cyclecar races before being signed to drive for Delage in 1924. The next year, teamed with Albert Divo, he won the French Grand Prix in the race that claimed the life of Italian racing star Antonio Ascari.

When the Delage company dropped out of racing, Robert Benoist was without a job and was appointed manager of the Banville Garage in Paris. He did occasional races for the Bugatti team, finishing second in the 1928 San Sebastián Grand Prix in Spain. The following year he teamed up with Attilio Marinoni to win the Spa 24 Hours race in Belgium, driving an Alfa Romeo. At the end of the season he retired until 1934, when he made a comeback with the Bugatti team. He was soon made head of the competition department and masterminded the company's Le Mans programme. In 1937 he partnered with Jean-Pierre Wimille to win the 24 hours of Le Mans[1] endurance race. Following that victory, Benoist retired permanently, but continued to run Bugatti's racing department until called up into the French Air Force.

Three days later, Robert Benoist was apprehended in Paris. While being driven to Gestapo headquarters, Benoist leaped from the moving vehicle and escaped,[3] eventually being smuggled back to England via the underground.

Benoist would later return to France on a second mission, lasting from October 1943 to February 1944, after which he returned to London for a short time before going back to France in March to work in the Nantes area with fellow SOE agent Denise Bloch.

Robert Benoist was arrested on 18 June 1944 and shipped to Buchenwaldconcentration camp where he was executed three months later, on 11 September.

In his honor, the village of Auffargis named a street after him and it is there in the churchyard cemetery on "Allée Robert Benoist" that fellow pioneer race driver, Ferenc Szisz is buried. Among the remaining grandstands still standing at the former Reims-Gueux circuit in France is one named "Tribune Robert Benoist".